[misc, android] Build and lint the mobile Go code in CI (#7641)

Nothing in CI compiles the files behind //go:build android or //go:build ios.
The android bridge builds 7 of its 23 files on linux and skips client.go; the
iOS SDK is not built at all. The linter matrix picks a GOOS by picking a runner
OS, so it loads the same file set as the host build and never sees them either.
A type error in client/android/client.go therefore passes every check on its
PR, merges, and is discovered by netbirdio/android-client after sync-tag.yml
fires trigger_android_bump on the release tag.

The new Mobile workflow cross-compiles ./client/android/... for the GOARCH
values gomobile ships and ./client/ios/..., and vets the android bridge. The
new Android and iOS lint jobs run golangci-lint with GOOS/GOARCH in the job
env. No NDK, Xcode or gomobile is needed: these are library packages, so the
compiler type-checks them without a link step, and the dependency graph drags
in the android/ios-tagged files across client/iface, client/internal/dns and
client/internal/routemanager with them.

Linting those files for the first time surfaces one gosec G101 on the SSH
password-required marker. It is a sentinel string the Java side matches on,
not a credential, so it is suppressed at the declaration.

.github/workflows/golangci-lint.yml

@@ -80,3 +80,49 @@ jobs:
           skip-save-cache: true
           cache-invalidation-interval: 0
           args: --timeout=20m
+
+  # Separate job rather than extra rows in the matrix above: those rows pick a
+  # GOOS by picking a runner OS, while android/ios are cross-compiled from
+  # ubuntu — an `include` entry with os: ubuntu-latest would merge into the
+  # Linux row instead of adding one. The package path is restricted because a
+  # whole-repo run under GOOS=android pulls *_linux.go files into packages that
+  # have no android counterpart.
+  golangci-mobile:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - goos: android
+            goarch: arm64
+            packages: ./client/android/...
+            display_name: Android
+          - goos: ios
+            goarch: arm64
+            packages: ./client/ios/...
+            display_name: iOS
+    name: ${{ matrix.display_name }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 25
+    env:
+      CGO_ENABLED: 0
+      GOOS: ${{ matrix.goos }}
+      GOARCH: ${{ matrix.goarch }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+      - name: Install Go
+        uses: actions/setup-go@924ae3a1cded613372ab5595356fb5720e22ba16 # v6.5.0
+        with:
+          go-version-file: "go.mod"
+          cache: false
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@82606bf257cbaff209d206a39f5134f0cfbfd2ee #v9.2.1
+        with:
+          version: latest
+          install-mode: binary
+          skip-cache: true
+          skip-save-cache: true
+          cache-invalidation-interval: 0
+          args: --timeout=20m ${{ matrix.packages }}

.github/workflows/mobile-build-validation.yml

@@ -0,0 +1,64 @@
+name: Mobile
+
+on:
+  push:
+    branches:
+      - main
+      - "release-*"
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.head_ref || github.actor_id }}
+  cancel-in-progress: true
+
+jobs:
+  android_build:
+    name: "Android / Build"
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        goarch: [arm64, arm, amd64, "386"]
+    env:
+      CGO_ENABLED: 0
+      GOOS: android
+      GOARCH: ${{ matrix.goarch }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+      - name: Install Go
+        uses: actions/setup-go@924ae3a1cded613372ab5595356fb5720e22ba16 # v6.5.0
+        with:
+          go-version-file: "go.mod"
+      - name: Build Android bridge
+        run: go build ./client/android/...
+      - name: Vet Android bridge
+        if: matrix.goarch == 'arm64'
+        run: go vet ./client/android/...
+
+  ios_build:
+    name: "iOS / Build"
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        goarch: [arm64, amd64]
+    env:
+      CGO_ENABLED: 0
+      GOOS: ios
+      GOARCH: ${{ matrix.goarch }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+      - name: Install Go
+        uses: actions/setup-go@924ae3a1cded613372ab5595356fb5720e22ba16 # v6.5.0
+        with:
+          go-version-file: "go.mod"
+      # No `go vet` counterpart: every ios target requires external (cgo)
+      # linking, which needs an Xcode toolchain the runner does not have.
+      - name: Build iOS SDK
+        run: go build ./client/ios/...